The hand plane is very important to woodworkers and furniture makers because it is the best tool for creating a finished flat surface in wood. Superior results are achieved with this tool because the wood fibers are cleanly severed, not torn, which creates a smooth, continuous surface ready for finishing. This is true even where the surface comprises two or more separate boards that have been joined side-by-side to create a larger piece.
The hand plane is so effective that it remains the tool of choice for final surfacing, despite all of the modern power tool alternatives. Power planers improve a wood surface, but leave noticeable and unattractive knife marks. Power sanders can remove the knife marks, but remove the knife marks, but tear, rather than cleanly sever, the wood fibers, resulting in a rougher surface.
Although today the body of a hand plane is more often made of metal, in earlier times these bodies were made of wood and the only metal portion would have been in the part incorporating the cutting edge. That part was termed the “iron” and the terminology continues even in speaking of modern planes that are substantially all metal.
In fine woodworking, the hand plane blade, or “iron,” used to impart the final surface finish is often curved or “crowned.” Such crowned irons cut a shaving from a wide board of wood, leaving no noticeable ridges in the wood because the shaving produced will be thickest along its longitudinal center line and gradually taper in thickness to nothing in the lateral directions, left and right of the shaving's center line. Thus the plane will not impart to the wood any “trails,” detectable by feel or appearance and the properly planed surface will be continuous and smooth so as to reveal the grain and color of the wood.
The crowned iron edge may have any of several geometries and still not leave noticeable trails through the planed wood surface. For example, when the plane holding the crowned iron is viewed longitudinally along the sole of the plane, revealing the slightly protruding iron, the effectively crowned edge may appear to have an edge shape that is circular, elliptical, or partially straight with rounded corners. The particular shape of crown is a matter of the woodworker's preference.
A crown is produced in the iron during sharpening after both first honing the iron's back flat and then honing the iron's bevel, typically at a thirty degree (30°) angle. The final step in sharpening the iron so that it is crowned involves stroking the bevel at the intended the intended bevel angle (again, typically 30°) while at the same time slightly varying the downward force applied to the iron laterally from side to side, in order to shift the point of the honing force continuously across the iron's width at the bevel during strokes along the longitudinal axis of both the stone and the iron. As may be evident, this manual operation requires a great deal of effort and skill in order to achieve a crown having the optimal geometry.
The afore-mentioned flattening of an iron's back is similar to the flattening of a chisel back or plane sole. Flattening is required on these tools whether they are new “out-of-the-box” or old, but especially if they are old and neglected. Once flattened, these tools stay flat if used with care. Maintenance of the flats is easier than the initial flattening operation. The flattening stroke is typically reciprocating or back-and-forth over a nominally flat stone with the tool's flat held against the stone.
Due to complex interactions of the human body's mechanics, the applied forces, and the dynamic forces of friction plus accelerations of mass, the stroking tends to introduce some rocking along with pure linear motion. The consequence of this rocking is the creation of some convexity in the tool surface that was intended to be flat. Such convexity may be lessened if considerable skill and technique are applied, but is extremely difficult to minimize, much less eliminate. One such technique is to manually dress the stone used for flattening against another stone or against a large, abrasive surface, such as a concrete sidewalk. By also using reciprocating strokes that similarly induce rocking of the flattening stone, the flattening surface becomes slightly convex. Although crude and imprecise, using such a slightly convex stone for flattening can, with care, reduce unwanted convexity in the iron back or plane sole.
After the iron's back flatness is checked and corrected as necessary, the sharpening activity shifts focus to the bevel. The angle of the bevel relative to the iron's back must be sufficient, given the iron's material strength, for the iron to accommodate the loading of the intended cutting when the iron is installed in the plane. For example, irons to be used to plane hardwoods may require a slightly greater bevel angle than those to be used to plane softwoods. Ultimately, experience should be the guide. Most sharpening starts at about 25° between the general plane of the iron and the sharpening stone or grinding wheel, with the previously mentioned flat back of the iron being held away from the stone or wheel. This initial angle of grind establishes the bevel on the iron.
Whether flat stones or grind wheels under power are employed to form the bevel is also a matter of preference. Using a wheel is faster and leaves a slight concavity in the bevel. Such “hollow” grinds weaken the iron, but also enhance feeling when the bevel is on a flat stone in subsequent steps. Rapidly rotating grinding wheels frictionally heat the iron and induce risk of removing the iron's tempered hardness. Today's diamond stones and certain abrasive stones are sufficiently fast acting that some practitioners do not use powered wheel grinding except for cases of chipped edges requiring extensive stock removal.
The above-mentioned rocking induced during stroking also applies to stroking a bevel on a flat stone. Flatness is desired on bevels because it creates an even cutting edge that achieves the desired result during use. One popular way to avoid convexity from being introduced during manual flattening of the bevel is to use a jig that clamps to the iron or chisel and provides more precise and constant bevel angle control. However, more time is required to set up the clamp and to adjust the bevel angle.
After the bevel is established, the sharpening progresses using successively finer-grained stones and a somewhat larger angle of approximately 30°. Emphasis is on creating a straight edge that is perpendicular to the longitudinal centerline of the iron. Also, the goal is to achieve an edge free of discontinuities, such as sharpening scratches that go through the edge from either the back or bevel side. As the bevel develops, a burr or so-called “wire edge” of very thin metal will form on the back side of the edge. At this stage, the edge may be either crowned and polished or just polished as a straight edge.
Traditionally, the crowning is done by a honing on the bevel with successive strokes, where the downward force applied to the iron against the sharpening stone is progressively off-center somewhat left, then full left, then to somewhat right, then full right, and finally shifted smoothly from one side to the other during a stroke. All the while the honing angle of about 30° is carefully held. With the crown established, bevel polishing is done with strop stokes on very fine waterstones or leather or lap surfaces, such as planed hard wood, medium density fiberboard (MDF), or flattened cast iron, that have been charged (typically with very fine abrasives such as rouge, chrome oxide, or diamond compounds). Strop strokes on the iron's back both wipes away any wire edge and polishes the iron up to the cutting edge.
What is needed is a sharpening means that achieves its desired effect regardless of the bevel angle or crown geometry, both of which optimally may be controlled as desired. Incorporation of diamond abrasive, yet enabling a controlled crowning stroke to be applied to a plane iron, would be particularly advantageous. Additionally, a sharpening apparatus that could be used to flatten the back, bevel, and/or sole of a woodworking tool, such as a plane iron or chisel, with minimal deviation from true flat would be highly desired. Finally, means desired. Finally, means for creating such sharpening apparati would also be desired.